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Sometimes evolution is stupid, and the human body is proof. Here are the most problematic physical and behavioral "scars of evolution" we humans have to deal with.

The human body is, in many respects, a resounding tribute to the adaptive powers of natural selection. We've evolved gloriously large, complex brains capable of abstract thought and foresight. We're bipedal, dextrous, and enjoy relatively long lives (lives that include a fairly generous fertility window), to list a few of the qualities that have allowed us to propagate and thrive across the planet.

But that doesn't mean we're perfect. Far from it, in fact. Not only did evolution create a species that's "good enough," it also produced some distinctly negative traits. Back in 1951, the biologist Wilton Krogman referred to these as the "scars of human evolution."

In some respects, these "scars" can be seen as vestigial traits, but that's not quite accurate. Rather, they're examples of the various trade-offs and side-effects of evolution. They're also not physical or psychological limitations per se (like our poor sense of smell or inability to grasp large numbers — those traits weren't adaptive in our recent evolutionary past). Also, I left out aging and age-related disorders; those aren't so much "flaws" of evolution as they're an indelible and insurmountable limitation of biological evolution. Lastly, I decided to include some characteristics that evolution designed rather poorly.

Without further ado here are humanity's most unfortunate design flaws; please add any that you think I overlooked to the comments.

Physical Characteristics

The Dual Function of the Pharynx

This is one of the most problematic "features" of the human body — and the cause of innumerable deaths throughout human history. Like many other primates, we're forced to use the same anatomical structure for both ingestion and respiration. But when obstructed, airflow is blocked, which can lead to choking, and in some cases, death.

Our Inability to Biosynthesize Vitamin C

Vitamin C plays a crucial role as an anti-oxidant and in collagen synthesis. But certain animals, such as primates, guinea pigs, and some bats and birds, have completely lost the ability to synthesize this compound. So, when Vitamin C-rich food sources are scarce, such as fruits, we experience a weakened immune response — not to mention scurvy in extreme cases.

Also, because we can't make all the vitamins we need, we carry a host of deadly bacteria in our bowels, which produce them for us. But when this process is disrupted, like a hole in the intestine, it can flare into peritonitis.

The Awkward Wiring of the Male Urinary Tract

The urinary tract in males passes through — rather than being routed around — the prostate gland, which can swell and block urinary function.The Close Proximity of our Genitals to our Rectum

Not only is this aesthetically displeasing, it's also unhygienic. Combined with our short urethras — especially in women — this leads to frequent urinary tract and bladder infections (UTIs) (remember, front to back, ladies). As the character Darald quipped in Forgetting Sarah Marshall, "Let me just say that if God was a city planner he would not put a playground next to a sewage system."

Our Multi-Function Genitals

Relatedly, our genitals are forced to perform multiple functions. While on the one hand it can be seen as conservation in design, it creates health problems. Again, it's unhygienic. For women, sexual intercourse pushes bacteria further into the urethra, leading to UTIs. Additionally, both men and women can contract UTIs from two sexually transmitted bacteria, chlamydia and mycoplasma. And of course, for women, this is also the part of the body where, in addition to sex and urination, newborn babies come out.

The Extremely Narrow Human Birth Canal

Speaking of which, human females have an unreasonably narrow birth canal, resulting in significantly increased risks to both mother and child during birth (not to mention prolonged labors and extreme levels of discomfort). In fact, death in childbirth used to be the leading cause of death for women during their reproductive years (our big, bulbous heads also has something to do with it). This is a consequence of our quick evolutionary leap from quadrupeds to bipeds, resulting in our narrow pelvis — the passage through which newborn babies pass.Our Over-Loaded Lower Backs

This is also a consequence of our transition from four-legged to two-legged creatures. According to paleoanthropologist Bruce Latimer of Case Western Reserve University in Cleveland, Ohio,

When humans stood upright, they took a spine that had evolved to be stiff for climbing and moving in trees and rotated it 90 degrees, so it was vertical—a task Latimer compared to stacking 26 cups and saucers on top of each other (vertebrae and discs) and then, balancing a head on top. But so as not to obstruct the birth canal and to get the torso balanced above our feet, the spine has to curve inwards (lordosis), creating the hollow of our backs. That's why our spines are shaped like an "S." All that curving, with the weight of the head and stuff we carry stacked on top, creates pressure that causes back problems—especially if you play football, do gymnastics, or swim the butterfly stroke. In the United States alone, 700,000 people suffer vertebral fractures per year and back problems are the sixth leading human malady in the world. "If you take care of it, your spine will get you through to about 40 or 50," said Latimer. "After that, you're on your own."

Our Achy Knees

Again, an aftereffect of bipedalism. We have to distribute all our weight on just two limbs, which often leads to aches and pains. You can also add achy, or arthritic hips, to this list.The Overly Complicated Human Foot

Anthropologist Jeremy DeSilva of Boston University put it this way:

Starting with the foot, DeSilva held up a cast with 26 bones and said: "You wouldn't design it out of 26 moving parts." Our feet have so many bones because our ape-like ancestors needed flexible feet to grasp branches. But as they moved out of the trees and began walking upright on the ground in the past 5 million years or so, the foot had to become more stable, and bit by bit, the big toe, which was no longer opposable, aligned itself with the other toes and our ancestors developed an arch to work as a shock absorber. "The foot was modified to remain rigid," said DeSilva. "A lot of BandAids were stuck on these bones." But the bottom line was that our foot still has a lot of room to twist inwards and outwards, and our arches collapse. This results in: ankle sprains, plantar fasciitis, Achilles tendonitis, shin splints, and broken ankles. These are not modern problems, due to stiletto heels; Fossils show broken ankles that have healed as far back as 3 million years ago.

A better design for upright walking and running, DeSilva said, would be a foot and ankle like an ostrich. An ostrich's ankle and lower leg bones are fused into a single structure, which puts a kick into their step—and their foot has only two toes that aid in running. "Why can't I have a foot like that?" asked DeSilva. One reason is that ostriches trace their upright locomotion back 230 million years to the age of dinosaurs, while our ancestors walked upright just 5 million years ago.

Our Inefficient Sinuses

Humans have several sinuses — air-filled cavities that help with drainage of mucus and fluid. But our maxillary sinuses, located on our cheekbone, drain upwards. This often leads to the build-up of fluids and mucus, which can cause an infection.The "Blind Spot" in Our Eyes

Our so-called "blind spot" is the result of a quirk that happens during embryological development. To deal with this, we've had to evolve elaborate and costly perception-correcting mechanisms. Our very own Esther Inglis-Arkell describes it like this:

Light gets into the eye by passing through the pupil. It hits the retina at the back of the eye. The retina is covered with light-sensing proteins. They relay what they sense to the optic nerve which carries the information back into the brain. The problem is, the optic nerve ends in the field of the retina itself. This is a little like having to plug the power cable for a TV directly into the screen. It creates a dark spot. Most of the time, the other eye will see what's happening in its partner's blind, but if the blind spots overlap while looking at a certain object, or if the person is only looking through one eye, the brain just fills in the spot looking at the surrounding picture.

A Single Set of Adult Teeth

This is where evolution got unreasonably cheap on us, providing humans with just one set of teeth for our entire adult lives. Once we hit 35, our teeth start to go — one of many signs that evolution primed us for reproduction, followed by a brief period of child-rearing, and pretty much nothing else (unless you subscribe to the grandmother hypothesis).

Behavioral Characteristics

Our fondness for sweet, salty, and fatty foods

Our bodies need sugar, salt, and fat — just not in extreme quantities. But in a state of nature, these foods are often scarce or difficult to preserve. That's why we find these food unreasonably delicious and irresistible. It's our body's way of saying, "Yo, eat this stuff while you can, it's good for you." But most of us now live in a world of tremendous abundance, and we consume these foods in ridiculous quantities, leading to all sorts of modern health problems.

Tribalism

Humans have a kind of ingrained fear or distrust of the "out-group." It's a previously adaptive trait that binds small groups of individuals together and prevents them from wandering off or joining other groups. But it also leads to ethnocentrism and divisions between groups. Studies show that oxytocin, while strengthening feelings of trust between individuals, increases fear of "the other." This characteristic was obviously important back when we lived in family clans or tribal arrangements, but today it leads to all sorts of social problems, including racism, prejudice, and our inability to empathize with people we don't immediately know.Any Number of Cognitive Biases

Many of our cognitive biases — annoying glitches in our thinking that cause us to make questionable decisions and reach erroneous conclusions — are a consequence of our limited intelligence and predisposed tendencies. Examples include the confirmation bias (we love to agree with people who agree with us), the gambler's fallacy (the tremendous weight we tend to put on previous events that aren't causal factors), our tendency to neglect or misjudge probability, and the status-quo bias (we often make choices that guarantee that things remain the same). Some of these are adaptive traits, but others are simply cognitive deficiencies.

Attention Deficit Hyperactivity Disorder

This is an example of how we've potentially pathologized a perfectly "normal" human psychological characteristic. Because ADHD appears to have a genetic component (it affects about 5% of school-aged children), questions have been raised about its prior role as a trait required for survival, namely its adaptive function in hunter, fighter, and wader theories. But today, we see it as something maladaptive — something that needs to be treated. Put another way, and like our penchant for sweet, salty, and fatty foods, it's a trait that's not so much nonoptimal as it's ill suited for present-day society.

Other sources: Better Than Human, WebMD, "Human Evolution: Gain Came With Pain".